There is a feature on page 6 of the Yorkshire Evening Post (YEP) today (Feb 14 2015) titled 'Six of the best hidden spaces' that has a brief report and picture of those "hidden world" places. All will have had mentions in SL (some extensively) but the feature is still very interesting. Of the tunnel (that with the two blue surface air vents) running from Northern Street to Queen's Street it mentions "its precise length and purpose appears to have been lost to memory". In a 'Queens Hotel and secret caverns' report it mentions that a tunnel "heads off under City Square and runs for a good 100ft, draped in silence and darkness, its floor made of uneven hardened mud and rubble". The other "subterranean secrets" sites featured are at Temple Newsam; Leeds station; Lawnswood and Oakwell Hall. If there is an online version of the feature I have not been able to find it.

Fine example of a YEP howler - the write up on their website (uncle mick's link above) carries the information that Leeds railway station's abandoned levels date back to the building of the original station in, apparently, the 18th century.